It is customary in many industries to protect given portions of a workpiece during a treatment process using shields, masks and similar devices. Such devices should be applied and removed with facility and provide adequate protection from the effects of the treatment process.
In the semiconductor industry portions of wafers have been protected using a variety of techniques, many of which have been adapted from the lithographic arts. Wafers are coated with an organic film and, after masking and exposure, unwanted portions of film are developed and removed. Thereafter an additive or subtractive process is utilized to create electrical devices and circuit paths in the wafers. Frequently, especially for integrated circuits, masks of oxides or nitrides are used to control subsurface diffusion of elements to alter the electrical characteristics of substrate materials. More recently, particle bombardment has been employed to create subsurface electrical conditions and masks have been developed to withstand such bombardment.
Masks of noble metals to protect current channels in a substrate from proton bombardment have been applied and removed by different techniques. In one technique, a layer of chromium is applied and a layer of gold is applied over the chromium and the gold is then selectively etched away by potassium iodide except over the channels. In another technique, grid wires of tungsten are applied along the current channels to absorb the effects of bombardment.
Neither of the above techniques has been ideal for masks which are readily applied and removed and which permit process adjustments. The tungsten wires are not readily varied in thickness to suit slight variations desired in the width of portions of a substrate to be protected. Gold strips are readily varied in width but removal of gold is sometimes difficult because solutions which dissolve gold are often harmful to substrate materials. For example, potassium iodide and aqua regia attack some substrates such as those containing gallium arsenide and others containing gallium phosphide.